Hi Jan,
On 5.6.2015 9:38, Jan Mareš wrote:
> Any input on that piece of code I sent in the previous message?
>
> Dne 4. 6. 2015 15:24 napsal uživatel "Jan Mareš" <[email protected]
> <mailto:[email protected]>>:
>
> I see, yes I realized my suggestion was wrong yesterday when I had a
> better look at it after I returned to my PC.
>
> Anyway, I promised you that I would try to reproduce the problems
> with threads and fibrils on plain fibrils. I think I have it, unless
> there is a bug in my code. Have a look at [1]. It's the smallest
> extract I was able to create to reproduce the race condition I'm
> running into. If I set PREEMPTIVNESS to 0 everything works fine, if
> I set to 1, I start to get page faults. It seems to me that
> async_futex is not doing it's job, given the stack traces from
> taskdump. But stack trace can be misleading as well.
>
> [1]
>
> http://bazaar.launchpad.net/~maresja1/helenos/qemu_porting/view/2212/uspace/app/posixtest/posixtest.c
It appears that the problem is caused by the fact that, for some reason,
there are not only two manager fibrils created, but literally hundreds
of them. And each of them ups the async_futex, rendering it thus useless
for mutual exclusion. So for example, in async_manager_fibril, I am
observing the up count for async_futex to be 180 and the number of
manager fibrils 338 and growing.
The respective code is here in fibril.c:
183 /* If we are going to manager and none exists, create it */
184 if ((stype == FIBRIL_TO_MANAGER) || (stype ==
FIBRIL_FROM_DEAD)) {
185 while (list_empty(&manager_list)) {
186 futex_unlock(&fibril_futex);
187 async_create_manager();
188 futex_lock(&fibril_futex);
189 }
190 }
and here in async.c:
1254 static int async_manager_fibril(void *arg)
1255 {
1256 futex_up(&async_futex);
1257
1258 /*
1259 * async_futex is always locked when entering manager
1260 */
1261 async_manager_worker();
1262
1263 return 0;
1264 }
Jakub
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